CMOS is a logic architecture that uses both PMOS and NMOS transistors to generate an output. The PMOS transistor and the NMOS transistor are connected in series. When the logic is not being changed, either the PMOS or the NMOS is open. As a result, when the logic is static, there is an open element in line which prevents the flow of current. During the transition between states, neither the PMOS nor the NMOS is open, so current flows and consumes power. When the logic changes state, for instance, goes from 0 to 1, both the PMOS and the NMOS change state in opposite directions, to assure that there is one open and one close element in series. In other words, if the PMOS changes from open to closed, the NMOS simultaneously changes from closed to open.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.